1. Field of the Invention
The present invention relates to image processing apparatus and particularly to image processing apparatus which provides an image signal for forming a hard copy image.
2. Description of the Related Art
In general, in the case of forming a full color hard copy image, the colors of an original image are reproduced in a process where color materials (such as toners or ink) of subtractive three colors, yellow (Y), magenta (M) and cyan (C) are caused to successively adhere on copy paper. The amounts of adhesion of the respective color materials are controlled by image signals.
If a color original image is decomposed into additive three primary colors, red (R), green (G) and blue (B), image signals corresponding to the respective colors of the color materials are produced from image data of the respective additive colors by sequential image processing (color correction processing) including black paint (BP) processing, under-color removal (UCR) processing and color correction masking processing.
As for the color materials, it is desirable for the color material of yellow (Y) to present a spectral characteristic (wavelength absorbing characteristic) absorbing only a blue (B) region, for the color material of magenta (M) to present a spectral characteristic absorbing only a green (G) region, and for the color material of cyan (C) to present a spectral characteristic absorbing only a red (R) region. However, in reality, the color materials do not exhibit such ideal spectral characteristics and unnecessary spectral absorption called sub absorption exists.
FIG. 25 is a graph showing general spectral characteristics of toners of Y, C, M used in color image formation in an electrophotographic process. The abscissa represents a wavelength of incident light and the ordinate represents reflectance corresponding to light absorption.
As shown in the figure, the Y toner exhibits a good spectral characteristic with high color purity but the C toner causes sub absorption in the G region and a little sub absorption in the B region. The M toner causes sub absorption in the B region.
Consequently, if overlap printing is effected by using those toners, muddiness occurs in the reproduced colors.
For example, if red (R) color is to be reproduced by mixing the M toner and Y toner, the Y component (the component absorbing the B region) in the M toner would be represented excessively or a yellowish muddy red color would be reproduced. Therefore, if the amount of adhesion of Y toner to be mixed with the M toner is decreased, an R region with high purity can be reproduced.
Such processing of adjusting a mixing ratio of image data corresponding to the respective colors of the toners is called color correction masking processing and this processing is indispensable for hard copy image formation.
In the case of color reproduction by successive overlapping of the color materials of three colors, it is difficult to reproduce pure black color and particularly in letter or fine line images, color deviation would often occur, causing the image quality to be lowered. Therefore, overlap printing using color materials of four colors including a black (Bk) toner in addition to Y, M, C toners is effected in normal color image formation.
The processing of producing image data of Bk from image data of the respective colors Y, M, C is called black paint (BP) processing, and in formation of the Bk image data, under-color removal (UCR) processing is applied to subtract a component compensated for Bk from the image data of the Y, M, C colors. By using the BP processing and UCR processing, it becomes possible to form an image of a high quality with excellent registration (mixing condition of colors) and the amount of consumption of the color materials of Y, M, C can be reduced.
The sequential color correction processing including the BP processing, UCR processing and color correction masking processing can be performed by calculation in a central processing unit (CPU). However, in an image reading apparatus where rapid processing is required for operation from reading of an original image to transmission of an image signal, hardware dedicated to color correction processing is used.
FIG. 26 is a block diagram of a color correction circuit 99 comprising a conventional image reading apparatus.
The color correction circuit 99 comprises: minimum image data selecting means 91 for selecting minimum image data Dmin out of the image data GT, BD, RD of the three additive colors; a multiplying device 92 for multiplying under-color removal coefficient data U by the minimum image data Dmin, thereby outputting under-color image data UD; a multiplying device 93 for multiplying black paint coefficient data K by the minimum image data Dmin, thereby outputting black paint image data BkD; UCR calculating means 94 for generating under-color removal image data Dg, Db, Dr corresponding to the respective additive color image data GD, BD, RD based on the under-color image data UD; selecting means 95 for selecting one of the under-color removal image data Dg, Db, Dr; and color correction masking means 96 for generating image data Yd, Md, Cd of subtractive colors based on masking coefficient data al, cl corresponding to the respective additive colors.
For example, if the image reading apparatus provides an image signal corresponding to the subtractive color Y, selection circuits 95g, 95b, 95r of the selecting means 95 select under-color removal image data Gd, Dr, Db as shown by the arrows in the figure. Thus, as a result of multiplication and addition operations in the color correction multiplying means 96, the generated image data YD is al.multidot.Dg+bl Dr+cl.multidot.Db.
The above described color correction circuit 99, where BP processing, UCR processing and color correction masking processing are performed by calculating means such as multiplying devices or adders, is capable of high-speed processing and has a low cost since a large-capacity table index ROM (Read Only Memory) is not required although complicated function calculation processing cannot be performed compared with a circuit using a table index system for color correction masking.
For purposes of making image reading apparatus compact and reducing the manufacturing costs, there are tendencies toward high degrees of integration in not only the color correction circuit but also other electric circuits in the apparatus.
To realize an integrated circuit, it is necessary to simplify the circuit configuration although it is also important to provide discrete circuits. For example, in the case of gate array which is advantageous for integration of digital circuits, it is important to perform operation processing with a smaller number of functional blocks in order to enhance the integration degree.
In some cases, a so-called monocolor image obtained by reproducing a full color original image by a single color is suited dependent on applications for which the image to be formed is used.
In view of such cases, many conventional image processing apparatus use a black paint image signal generated by the BP processing and output the same in substitution for a monocolor image signal. In addition, as described for example in Japanese Patent Laying-Open No. 62-116958, a monocolor image signal is formed by compositing image data of respective subtractive three colors Y, M, C formed by color correction masking processing.
The black paint image signal is primarily an auxiliary signal formed to enhance reproducibility of pure black color and the registration (color mixing degree) in reproduction of letter or line images and to adjust the tone of a high density portion. In addition, the black paint image signal has difficulty in adjustment with the color materials of subtractive three colors if a Bk color material is used for the entire density region of an original image, and fouling with the Bk color material is liable to occur especially in a bright portion. Consequently, the black paint image signal is generated based on a skelton black method where the mixing ratio of the Bk color material increases according to increase of the density.
Thus, in the conventional apparatus using the black paint image signal in substitution for a monocolor image signal, a monocolor image of high quality cannot be formed with good balance over the entire density region.
If a monocolor image data is to be composited from image data of the three subtractive colors, the quality of the monocolor image can be improved by appropriately setting the mixing ratio of the three colors. For that purpose, it is necessary to provide a high-speed and large-capacity ROM which stores composite data for all of values of the image data of the three colors. Generally, the color correction processing means is formed by four-operation means including combination of logical elements and if a ROM having a quality different from the logical elements is used, it is not possible to integrally form the dedicated hardware for color correction processing, making the entire structure of the image processing apparatus complicated.
An image processing apparatus incorporated in an image forming apparatus (such as a digital copying apparatus, a facsimile, and various printers) for forming a hard copy image of an original image on an original or a CRT display, image processing called density correction processing is performed with respect to image data corresponding to respective pixels of the original image so that the hard copy image of a desired density can be formed.
Density correction in a conventional image processing apparatus is carried out by a table index system (lookup table system) as described for example in Japanese Patent Publication No. 60-23541.
More specifically, density correction image data is stored in advance in a ROM, and density correction image data of an address designated by bit data corresponding to a designated density and by the image data before correction is outputted as an image signal.
According to the table index system as in the prior art, an arbitrary correction pattern can be set according to the content of the density correction image data prepared in the ROM.
However, in order to attain fine adjustment close to stepless adjustment by increasing the number of density inclinations to be designated, namely, the number of steps of density designation such as degrees light.fwdarw.slightly light.fwdarw.normal.fwdarw.slightly thick.fwdarw.thick, it is necessary to provide a large-capacity ROM, causing increase in the size of the apparatus.
In addition, a high-speed ROM integrated circuit device, with access time of 50 nsec or less is expensive at present and the manufacturing cost of the device comes to be high.
On the other hand, according to the table index system, an arbitrary correction characteristic can be set according to the content of density correction image data prepared in a ROM. For example, if a correction characteristic is set to increase or decrease a value only with respect to image data of a prescribed density or more, no fouling occurs in the background area (white area) of the original image even if the density is increased.
However, once the content to be stored is set, output image data (density correction image data) is definitely defined with respect to input image data and if the background color of the original image has a density higher than a set sensity, that is, if the original image is formed on colored (other than white) background such as blue print paper or newspaper, the density of the background color is also corrected together with the density of the image, causing an unclear image of a deteriorated contrast between the background color and the image.
Thus, in density correction according to the table index system, contrast adjustment between the background area and the image area and density adjustment of the image area cannot be performed independently.
Therefore, it may be considered to use a method of correction of image data (background correction) to attain a good contrast with the background color by increasing the intensity of light of a light source illuminating an original to a value larger than that for white original according to the density of the background area, or adjusting a reference potential for A/D conversion in quantization of a photoelectric conversion signal obtained by reading the original. However, such method involves analog control and a stable precision cannot be obtained. In addition, the dynamic range of the image data is changed due to the correction, making it difficult to set a correspondence with density correction by image processing (digital signal processing).
In addition, in the conventional image processing apparatus, density correction processing for forming a hard copy image of a desired density is performed after image data corresponding to the respective colors of color materials are converted to image data corresponding to a color edited image.
Consequently, the density of the image formed is controlled indiscriminately and the density of a designated area for example to be painted out is also changed together with other areas.